The invention relates to friction gears in general, and more particularly to improvements in so-called continuously variable transmissions.
A continuously variable belt or chain transmission normally comprises a rotary input shaft which carries a first sheave, a rotary output shaft which is parallel to the input shaft and carries a second sheave, and an endless belt or chain which is trained over the sheaves. Each sheave comprises a first flange which is rigid with the respective shaft and a second flange which is movable axially of the respective shaft toward or away from the corresponding first flange. The transmission further comprises means for moving the axially movable flanges relative to the associated first flanges. Reference may be had to German Pat. No. 28 28 347 wherein the force acting upon the movable flange of the sheave on the input shaft is varied in dependency on transmitted torque by a torque sensor which is or which includes a valve adapted to be adjusted as a function of changes of transmitted torque. The valve controls the flow of a fluid which is supplied by a separate pump and is designed to close, at least in part, when the sensor detects an abrupt rise (peak) of transmitted torque. This entails a corresponding rise of pressure in the cylinder and piston units which are used to change the positions of the axially movable flanges. Consequently, the force with which the flanges of the two sheaves bear upon the adjacent portions of the endless belt or chain increases proportionally with increasing torque. The means for adjusting the valve in response to fluctuations of torque comprises two confronting discs which are provided with cams and bear upon interposed rolling elements. The rolling elements are acted upon by the discs with a force which is proportional to fluid pressure in the valve and to the pressure at the outlet of the pump. When the torque increases as a result of a surge at the input side, the two discs are caused to move apart and one of the discs then reduces the rate of flow, or interrupts the flow, of fluid through an outlet port. The rate of flow of fluid through the port is altered proportionally with the surge of transmitted torque. The discs further serve as a means for mechanically transmitting torque as well as to adjust the valve accordingly, i.e., to reduce the rate of fluid flow through the valve proportionally with an increase of transmitted torque. This entails a corresponding change of the force which the axially movable flanges of the sheaves apply against the adjacent portions of the flexible element. The arrangement is such that the valve normally establishes a path for the flow of hydraulic fluid therethrough (the valve is closed only in response to the development of highly pronounced peaks of transmitted torque). Therefore, the pump must be capable of supplying fluid at a rate which is necessary to establish and maintain a flow through the valve at a rate which is proportional to the magnitude of transmitted torque as well as to ensure that the discs are properly biased against the rolling elements between them, i.e., that the discs can transmit torque to the input member of the transmission. Thus, the quantity of fluid which escapes by way of the normally open valve as well as a result of inadequate sealing is rather high. The fluid is permitted to leak via valve and/or elsewhere at all times, i.e., as long as the transmission is in actual use.